It sounds like a spot of gallows humour, but the numbers are no joke: the US environmental protection agency (EPA) has lowered the value of a human life by nearly $1m under George Bush's administration.

The EPA's estimate of the "value of a statistical life" was $6.9m as of this May – down from $7.8m five years ago – according to an Associated Press study released today.

Though it may seem like a harmless bureaucratic recalculation, the devaluation has real consequences.

When drawing up regulations, government agencies put a value on human life and then weigh the costs versus the lifesaving benefits of a proposed rule.

The less a life is worth to the government, the less the need for a regulation – such as the tighter restrictions on pollution that the EPA refused to impose today, effectively postponing any action on climate change until after Bush leaves office.

Consider, for example, a hypothetical regulation that costs $18bn to enforce but will prevent 2,500 deaths. At $7.8m per person (the old figure), the lifesaving benefits outweigh the costs. But at $6.9m per person, the rule costs more than the lives it saves, so it may not be adopted.

Some environmentalists accuse the Bush administration of changing the value to avoid tougher rules, a charge the EPA denies.

"It appears that they're cooking the books in regards to the value of life," S William Becker, executive director of the National Association of Clean Air Agencies, said.

"Those decisions are literally a matter of life and death."

Dan Esty, a senior EPA policy official in the administration of the first President Bush and now director of the Yale centre for environmental law and policy, said: "It's hard to imagine that it has other than a political motivation."

"EPA may not think Americans are worth all that much, but the rest of us believe the value of an American life to our families, our communities, our workplaces and our nation is no less than it has ever been," Boxer, a Democrat, said.

Agency officials say they were just following what the science told them. The EPA figure is not based on people's earning capacity or their potential contributions to society -- some of the factors used in insurance claims and lawsuits.

Instead, economists calculate the value based on what people are willing to pay to avoid certain risks, and on how much extra employers pay their workers to take on additional risks.

Most of the data is drawn from payroll statistics; some comes from opinion surveys. According to the EPA, people shouldn't think of the number as a price tag on a life.

Vanderbilt university economist Kip Viscusi, whose work was used by the EPA in evaluating whether to lower the value of a life, said the cut "doesn't make sense".

"As people become more affluent, the value of statistical lives goes up as well. It has to," Viscusi told the Associated Press. He also said no study has shown that Americans are less willing to pay to reduce risks.

The EPA traditionally has put the highest value on life of any government agency and still does, despite efforts by past administrations to use the same figure in all US government agencies.